An expected energy model works out how much energy or power a PV system should produce under the weather actually measured on site, such as irradiance and module temperature. The model first finds the effective plane-of-array irradiance, applies shading corrections, and then calculates the DC power available from the modules. Next it applies DC-side losses (age, derate, wiring, LID and mismatch), then inverter conversion losses and clipping, and finally a scaling factor for losses it does not model explicitly, such as AC wiring and transformer losses. Expected energy includes the design losses built into the model, such as shading and inverter efficiency, but it leaves out soiling, snow, curtailment and downtime, so those losses show up as gaps against measured energy. Expected energy is not the same as estimated energy: estimates are monthly budget values based on long-term averages and do not depend on measurements. Dividing measured energy by expected energy gives the energy ratio or performance index, which should be close to one for a healthy system. A simpler option is a regression equation in irradiance and temperature, fitted to measured data and used for commissioning and long-term O&M baselines. Results are only as accurate as the tuning of the model and the quality of the irradiance and temperature data feeding it.
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related_toc1 drafted · lead Expected energy ratio
The energy ratio divides measured energy by expected energy and should be 1.0 for a system operating normally.
sec=front h=95bd390c335fsec=front h=003faad8cb7cc2 drafted · lead Insolation-adjusted expected energy ratio
IEER uses expected AC energy (Ee) and adjusts it by the ratio of measured to expected insolation.
sec=front h=c83c6d979313sec=front h=003faad8cb7cc3 drafted · lead Performance index
Performance index is measured energy divided by modeled energy.
sec=front h=003faad8cb7creview note: support check: partial - Performance Index is Measured/Modeled energy per passage, but the link to the expected energy model is only via 'modeled energy may refer to expected energy'; the waterfall article even states it inversely.
c4 drafted · lead Energy loss waterfall
The waterfall connects estimated energy to expected energy, then expected energy to measured energy.
sec=front h=b18a92c492b3review note: support check: partial - Waterfall shows Estimated→Expected and Expected→Measured, but the relation to the expected energy model is implicit.
c5 verified · fact Plane-of-array irradiance
The model's first step is to work out the effective POA irradiance.
sec=front h=1c0c0061a1d8sec=front h=53994d8c99ffverified by: review-workflowverified on: 2026-10-24
c6 verified · fact Irradiance transposition
Transposition is needed when the sensor is not mounted in the same orientation as the array.
sec=front h=1c0c0061a1d8sec=front h=53994d8c99ffverified by: review-workflowverified on: 2026-10-24
c7 drafted · lead Clear-sky irradiance model
sec=front h=95bd390c335ftbl=1.r3 h=b785dd8cf306review note: support check: partial - Passages define the clear-sky model and mention blue-sky expected output in the PV system model, but do not clearly state a relation to the expected energy model.
c8 drafted · lead Typical meteorological year
In the IEER formula, the long-term expected energy and expected insolation are based on TMY data.
c9 drafted · lead AlsoEnergy PowerTrack platform and API
sec=front h=1c0c0061a1d8sec=h1 h=efbe446c42daA shading map generated in PVsyst can be uploaded, and the inter-row shading uses the same parameters as the PVsyst Sheds model.
sec=front h=1c0c0061a1d8sec=front h=53994d8c99ffverified by: review-workflowverified on: 2026-10-24
c11 drafted · lead Energy ratio analysis
tbl=13.r4 h=729bb0c18bactbl=13.r5 h=cd94e1ad1a36degradation_ratec12 drafted · lead nominal 0.5 PERCENT-PER-YR
Conditions: Commonly used default PV system degradation value in the PowerTrack expected energy model; annual compound percentage is the default mode
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